The present invention relates to liquid-liquid extraction involving the dispersion of a relatively heavier liquid phase in a relatively lighter liquid phase, at elevated ratios of flow rates of light phase to heavy phase. The present invention also relates to mechanical improvements in liquid-liquid extraction equipment.
In a typical form, liquid-liquid extraction processes perform mass transfer of target components from one liquid phase into a second liquid phase, typically to selectively recover valued components or eliminate undesirable components from one phase. The liquid phase containing the components to be extracted is called raffinate. The liquid phase that will extract the target components is called solvent, and when the solvent has completed the extraction, the solvent phase is called extract.
In liquid-liquid extraction applications, two liquid phases are characterized by, among other properties, different relative solubilities of the target components being extracted, and different bulk densities. The two liquid phases must be brought into intimate contact with each other to make the mass transfer process efficient. A typical method of achieving intimate liquid-liquid contact entails breaking up a light liquid phase into small droplets and dispersing those droplets into a heavy liquid that is sustained as a continuous phase. This can be achieved using, for example, perforated sieve trays arranged successively in a preferably vertical liquid-liquid extraction vessel. In such a system the light and heavy liquids are typically introduced near bottom and top ends, respectively, of the extraction vessel, and the liquids flow countercurrently past and through one another because of differing respective liquid weights.
Prior-art liquid-liquid extraction applications in sieve tray systems typically disperse the light liquid phase as droplets, as the light phase rises through sieve tray perforations. The light phase droplets pass through a continuous, heavy liquid phase above each sieve tray. This physical arrangement can be appropriate for applications in which light-phase flow rates are roughly equal to or substantially lower than heavy phase flow rates. The practice of dispersing a rising light phase also has been particularly favored to minimize maintenance and capacity problems associated with solids that may settle out of either liquid phase and block tray perforations by coming to rest on the tray surfaces.
U.S. Pat. No. 4,247,521 to Forte et al discloses a method of liquid-liquid extraction in terms of applications to systems characterized by substantial excess in the flow of the heavy phase over that of the light phase. Forte et al also features increasing tray riser complexity as vessel and tray diameter increase.
U.S. Pat. Nos. 5,047,179 and 5,049,319 to Nye address increasing active sieve tray area while also adding to vapor flow area to promote phase disengagement in vapor-liquid applications such as distillation. This can improve both fluid throughput capacities and vapor-liquid contact efficiency, for net gains in unit capacity and separation effectiveness.
Bravo et al, “Sulfolane® RDC Tray Revamp,” AlChE Meeting, Chicago, Ill., November 1996, presents a case of retrofitting a liquid-liquid extraction vessel to replace the internals of a rotating disc contactor (RDC) with fixed sieve trays having five upcomers per tray.
Zhu et al, “Hydrodynamic and Mass Transfer Performance of Multiple Upcomer Extraction Trays,” Canadian Journal of Chemical Engineering, August 1997, describes laboratory work investigating operation of sieve trays with three upcomers (risers).
Each of the above patents and references is hereby incorporated by reference in its entirety.
In the particular case of deasphalting lubricating oils, a rotating disc contactor (RDC) has been commonly used. RDC's have evidenced operating problems such as backmixing or premature flooding, which can be caused by excessive shear of the rotating discs that can create too fine a dispersal of process liquids. Results of very fine dispersion in a liquid-liquid extraction unit can include internal recirculation or entrainment of the dispersion. Both phenomena reduce extraction efficiency and capacity. Lube oil deasphalting typically operates at elevated pressures that may equal or exceed supercritical pressure, and RDC's have been prone to seal leakage where disc drive shafts penetrate the extraction vessels. In a number of RDC units seal leakage has been stopped or limited by welding the disc drive shaft seals, thus abandoning any advantages of agitation to enhance extractive mass transfer, and thereby limiting such columns to perform either at reduced throughput or reduced separation, or both.
Liquid capacity, extraction efficiency, and product yield of RDC units are affected by excessive shear, with the potential impacts of light-phase backmixing and/or heavy-phase entrainment. Both phenomena interfere with phase separation and cause unproductive secondary phase contact, thus reducing extraction efficiency. One way to counteract backmixing and entrainment is to reduce liquid throughput rates, allowing added residence time for phase separation, but also reducing column capacity. Any of the three results i.e. operating with backmixing or entrainment, or operating at reduced liquid capacity reduces product yield.